When the dentist is restoring a tooth, the cavity made is cleaned and thereafter insulated. As insulation so called "cavity liners" are used. These cavity liners have been thick and cement-like. They present many inconveniences, such as e.g. the fact that they cannot be applied but to the floor of the cavity. Since they do not cover the important sidewalls, they do not reduce the risk of secondary caries. Furthermore, they are voluminous and can gradually be entirely dissolved. Some of them even irritate the pulp.
It is also known that varnish, e.g. a vehicle together with a solvent, has been tried as insulation agent. The insulation obtained has not proved to be completely satisfactory, particularly not as to bacterial invasion. Furthermore, there will be a space between the cavity walls and the insulation, which moreover does not prevent growth of existing bacteria.
Recently cavity liners have been used consisting of a resin dissolved in chloroform together with a suspension of some other substances, particularly salts. The resin employed has especially been polystyrene and the suspended substance employed has been Ca(OH).sub.2 and/or ZnO. In other types of cavity liners calciummonofluorophosphate and diiodide dithymol have been used. Such cavity liners have proved to provide a good protection to most filling materials, particularly if they have been applied under specified conditions, in which case, however, certain clinical difficulties exist.
The principal object of the insulation is to protect against growth of bacteria in the space between the filling and the cavity wall and to prevent bacterial invasion from the oral cavity and to prevent toxins from diffusing through the dentinal tubules to the pulp. The following requirements should therefore be put on a liner:
it should not be tissue irritating;
it must be easy to apply the means of an air-blast to a thin and uniform film (thickness about 3-10 .mu.m), which well covers the whole surface of the cavity;
only one application should be sufficient for achieving a satisfactory protection;
a good adaptation should take place to ground dentinal and enamel surfaces which have been cleaned and dried with an air blast (with a remaining monolayer of fluid);
it should have bacteriostatic properties;
it should protect against bacterial invasion in the space between filling and cavity wall; in other words, the liner should be of such a nature that it, when absorbing fluid, (especially from the dentin, where tissue fluid exists and all the time provides pressure from the inner of the tooth) expands so that a contraction space of about 1-10 .mu.m as far as possible is filled by the liner, said contraction space being the result of the hardening of the filling material, and that it is not dissolved to any greater extent by oral liquids and acids, which can be formed in bacterial plaques in connection with the outer limit of the filling;
it should not be appreciably influenced by a short contact with 30-50% by weight phosphoric acid, followed by water flushing.
It has been proved that salts, which are found in the prior known liner, should be part of the suspension. A smaller amount of Ca(OH).sub.2 in a thin plastic film does not apparently bring about any detectable positive effect as to dentin. The same applies to smaller amounts of fluoro containing salts.
Thus, in prior known cavity liners chloroform is used as a solvent for the resinous materials (plastic materials) used, which can be polystyrene, polyacrylate and similar resins, for instance. However, it has now been known that chloroform as a solvent presents a number of inconveniences. Thus, it is now suspected to have cancerogenic properties. Furthermore, it is poisonous.